Emergency Contraception & Irregular Periods: Seeking Information

Question
Hi I have a few questions that I would like to ask you. First a little background on the situation. In December of 2006 my fiance/boyfriend of 6 years, we broke up. On December 20th I had relations with another man and proceeded in taking the morning after pill on December 22nd. On January 7-9th I had a menstrual (a constant flow a little lighter then usual a day shorter, keep in mind my menstruals are often irregular).  In February I didn't have a menstrual so on February 25th i took a pregnancy test which came back positive. I then went to the doctor on February 26th which they did an ultrasound that showed me with a gestational age of 7 weeks and 3 days. To further inform you..my period prior to January was in December on the 10th I believe and also my fiance and I got back together on Christmas and have been sexually active ever since.  So my questions are 1) how accurate are ultrasounds in early first trimester? 2) is there any possible way that the baby could be the other mans? 3) could emergency contraceptive have caused my menstrual to be a little bit different in January.

Answer
Dear Brandi,

OK - I'm going to approach the questions backwards...
First, absolutely the EC can and most likely would have caused an unusual period. It's a very common side effect.

The most accurate ultrasounds, for dating purposes, are the early ultrasounds. Ultrasounds done after the first trimester, especially after 20 weeks, begin to be affected by differences in each baby's growth. So, an ultrasound done at 7 weeks would have been quite accurate.

Now remember, gestational age is determined by LMP. Actually, the ultrasound and the LMP history confirm each other. By my calculation, your conception date would have been approximately 1/20-1/22.

This would have been well after the single encounter with the other man in December. If that had been the conception date, you would have been more like 11 weeks at that first ultrasound.

I hope this has helped you and answered your questions. I apologize for the delay in the answer - I was on a rare vacation for the 4th and for some reason this system didn't schedule it as it should have.

Brenda